Quantifying the chemistry of sulfide in the core and its influence on the composition of the silicate Earth

Lead Research Organisation: Durham University
Department Name: Earth Sciences

Abstract

The chemical composition of the Earth and rocky planets of the inner solar system is traditionally thought to be the same as primitive "chondritic" meteorites, amongst the earliest material formed in our solar system. Recent work, however, shows that the Earth does not have a chondrite-like composition, but instead appear to have lost a substantial part (some 10%) of its mass early in solar system history. The material that has been lost is highly enriched in so-called "incompatible" elements (that is, those elements with a large size (ionic radius) that preferentially go into silicate melts) including uranium, thorium and potassium, which together are responsible for generating much of Earth's internal heat through radioactive decay.

The isotope signature observed in the modern silicate Earth indicates that this material must have been lost from Earth in the first 100 million years of its history. During this time metal separated from silicate in the growing Earth, forming a metallic core at the centre of the planet, leaving a silicate mantle above and crust at the surface. Some have argued that the loss of "incompatible" element rich material this was due to the removal of Earth's earliest crust through collisions with other growing planets, but this leaves the Earth without its full complement of heat-producing elements. Others have argued that these elements might be stored in a hidden reservoir deep in the silicate mantle, but so far no chemical or thermal trace of this reservoir has been observed. Moreover, models suggest that a high concentration of heat-producing elements at the base of the mantle may prohibit a functioning geodynamo (generation of the Earths magnetic field in the liquid outer core). Each of these hypotheses has very different implications for the chemical, dynamic and thermal evolution of Earth, but each poses problems that are difficult to circumvent.

Seismic data from earthquakes and experimental work indicates that the Earths metallic core is principally composed of Fe-Ni metal, but also includes "lighter" elements, chief amongst which is sulphide. Experimental and isotope data suggest that sulphur was added late to the core either as a S-rich metal or as sulfide. Normally the "incompatible" are not expected to be incorporated into Fe-Ni metal, however, remarkably our own preliminary experimental data indicate that they are enriched in sulfide. At the same time new stable isotope data are also consistent with the incorporation of "incompatible" into sulfide and subsequent migration to the core. That sulfide in the core provides an incompatible element enriched reservoir capable of balancing the composition of the silicate Earth, offers an elegant solution to the non-chondritic Earth. At once reconciling the problem of planetary depletion of the heat-producing elements and providing a heat-source for the geodynamo.

The overall aims of this project are (1) to quantify the role of sulfide during Earth's growth and core formation through high-pressure experiments that simulate the conditions of core formation. (2) Assess the influence of sulfide in the core on the composition of Earth's silicate mantle, and (3) the potential influence of continent formation and recycling using neodymium stable isotopes.

Planned Impact

The Earth Science departments in Durham and Oxford have an established track record of effective engagement with a wide spectrum of beneficiaries, from the public, to academia to industry - and with the work proposed here we will build on that experience.

Recent activities include: Hosting workshops for industry and policy makers; engagement with the media (print, radio and television), outreach activities in schools (talks and practical classes) and with the wider public (Royal Society summer exhibition, departmental open days and the annual NERC science open day). Both Durham and Oxford have effective press offices - with regular press releases and widely accessed, frequently updated, websites.

The results from this work will be of interest not only to the part of the academic community engaged in understanding the construction and chemical evolution of the earth and other planets, but also to the commercial private sector, both in terms of understanding the some of the controls on natural sulfide in continental rocks, in particular copper-porphry deposits in the continents, and the industrial production of synthetic sufides for a wide range of applications. Finally the coupled application of state-of-the-art experiments and with novel isotope analysis goes far beyond the fields of geochemistry and Earth Sciences, with potential applications in medical and life sciences, and the growing field of nano-particle technology.

The recent interest in the Rosetta mission to the comet 67P/Churyumov-Gerasimenko has highlighted the appetite of the general public for blue-skies research, inspiring school children across the World, facebook sites, blogs and a public debate on the origin of Earth's water. To maximise the impact of the proposed research across scientific disciplines to a wider public audience it will be necessary to make our results comprehensible to those with a limited geological expertise, both scientists and the public alike. Both in Durham and Oxford, this work will play a major part of the NERC Science Open Day and the annual National Science week activities, and will be used to develop activities that engage students and the wider public both in the ways that scientific research actually works and the types of information that it may provide.

Publications

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McCoy-West A (2017) The neodymium stable isotope composition of the silicate Earth and chondrites in Earth and Planetary Science Letters

 
Description Systematic variations in the Nd stable isotope composition of the Earth and primitive solar system materials. Experimental constraints on the fractionation of Nd stable isotopes between sulphide and silicate melt
Exploitation Route Understanding planetary accretion and core formation, continent formation on Earth, partitioning of the REE and other incompatible elements in sulphide
Sectors Chemicals,Environment

 
Description AGU Conference, New Orleans, December 10-14, 2017 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Research presentation at major International Conference
Year(s) Of Engagement Activity 2017
 
Description Goldschmidt conference, Boston, August 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Presentation of research results
Year(s) Of Engagement Activity 2018